Non-aqueous electrolyte secondary battery

ABSTRACT

A non-aqueous electrolyte secondary battery including: an electrode group in which a positive electrode and a negative electrode are spirally wound with a separator interposed therebetween; and a non-aqueous electrolyte including a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent, the positive electrode including a positive electrode material mixture layer containing a nickel-containing lithium composite metal oxide, wherein a product of A and B equals 150 to 350, A equals 15 to 20%, and B equals 10 to 25%, where A (%) represents a porosity of the positive electrode material mixture layer, and B (%) represents a volume percentage of ethylene carbonate in the non-aqueous solvent.

FIELD OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery, and more particularly to an improvement of safety.

BACKGROUND OF THE INVENTION

With the widespread of smaller and lighter weight electronic equipmentsuch as cell phones and notebook computers in recent years, demand isgrowing for higher capacity secondary batteries as the power sourcestherefor. Non-aqueous electrolyte secondary batteries, which include: apositive electrode whose active material is a lithium cobalt oxide(e.g., LiCoO₂); and a negative electrode comprising a carbon material,have been developed for such application and are now widely used.

LiCoO₂, however, is very costly since it contains Co. For this reason,as an alternative to LiCoO₂, various other metal oxides are proposed andvigorously studied. Examples of such metal oxides include: LiNiO₂;LiNi_(1-x)Co_(x)O₂ obtained by partially replacing Ni in LiNiO₂ with Co;and LiMn₂O₄.

Particularly, a positive electrode whose active material is a lithiumcomposite oxide containing nickel as an essential element (hereinaftersimply referred to as a nickel-containing oxide), such as LiNiO₂ andLiNi_(1-x)Co_(x)O₂, can offer higher energy density than a positiveelectrode whose active material is a lithium cobalt oxide. Accordingly,the use of positive electrode comprising a nickel-containing oxideenables low cost production and provides a non-aqueous electrolytesecondary battery with improved capacity characteristic.

However, the positive electrode comprising a nickel-containing oxide haslower thermal stability than the positive electrode comprising a lithiumcobalt oxide, and thus the resulting battery has the disadvantage ofpoor safety.

As one approach for improving safety, Japanese Laid-Open PatentPublication No. 2000-195557 (hereinafter referred to as PatentDocument 1) proposes a battery that satisfies a relation: 0.2<C3/C1<0.8in a region of 45<P/S, where P (mAh) represents battery nominalcapacity, S (cm²) represents battery surface area, Cl (mAh) representsdischarge capacity discharged at P (mA), and C3 (mAh) representsdischarge capacity discharged at 3×P (mA). This publication alsoproposes to produce a positive electrode whose active material layer hasa density of not less than 3.2 g/cm³ in order to achieve the aboverelation.

As another approach, although silent on positive electrode activematerial, Japanese Laid-Open Patent Publication No. 2000-123870(hereinafter referred to as Patent Document 2) proposes a solvent fornon-aqueous electrolyte comprising ethylene carbonate and methyl ethylcarbonate at a volume percentage of not less than 10% and not greaterthan 30%, and not less than 50% and not greater than 90%, respectively.According to this publication, when the content of ethylene carbonate isless than 10%, the effect of forming a protection film on the surface ofnegative electrode active material is reduced. When the content ofmethyl ethyl carbonate, which has low viscosity and a low boiling point,exceeds 90%, the resulting battery may generate heat due to ashort-circuit, increasing the possibility of explosion.

It is generally accepted that the positive electrode whose activematerial is a nickel-containing oxide containing nickel as an essentialelement lacks safety because the active material has poor thermalstability. If a short-circuit occurs inside a fully charged battery, alarge current will flow locally into the shorted area, therebygenerating heat due to Joule heat.

In nail penetration test (one of the safety tests) in which a nail ispenetrated through a battery, the most dangerous situation occurs when apositive electrode current collector and a negative electrode materialmixture layer come into contact with each other. When a nail ispenetrated through a battery, the positive electrode active materiallayer separates from the positive electrode current collector at thepenetrated area, allowing the bare current collector to come intocontact with the negative electrode active material layer through thenail, causing excessive heat generation.

In a battery comprising a nickel-containing oxide, it is difficult toprevent excessive heat generation under such conditions as in nailpenetration test only by increasing the density of the positiveelectrode active material to a certain level as disclosed by PatentDocument 1. Moreover, as described in Patent Document 1, increasing thedensity of the positive electrode active material results in a decreasein high rate discharge performance.

Likewise, only combining a battery whose positive electrode activematerial is a nickel-containing oxide with the electrolyte disclosed byPatent Document 2 (i.e., the electrolyte comprising ethylene carbonateand methyl ethyl carbonate at a volume percentage of not less than 10%and not greater than 90%, respectively) cannot prevent excessive heatgeneration. Nickel-containing oxides have the disadvantage of lowthermal stability. Although the cause has not been determined, thefollowing inherent characteristics of nickel-containing oxides arepresumed responsible for low thermal stability. Specifically, a highvalence metal oxide decomposes at high temperatures and releases oxygen.The thermal decomposition temperature of positive electrode activematerial tends to be influenced by the charge state of battery (i.e.,the amount of lithium contained in positive electrode). In other words,positive electrode active material tends to decompose more easily as theamount of lithium is reduced. In a nickel-containing oxide, because theamount of lithium is small, the crystal tends to be unstable. When anickel-containing oxide and a cobalt-containing oxide both having anequal amount of lithium are compared, nickel-containing oxide isthermodynamically more unstable and thus easily releases oxygen.

SUMMARY OF THE INVENTION

An object of the present invention is to improve a non-aqueouselectrolyte secondary battery comprising a positive electrode containinga nickel-containing oxide so as to ensure its safety without sacrificingits high rate discharge performance, thereby providing a battery whichis excellent in safety and battery characteristic.

A non-aqueous electrolyte secondary battery of the present inventioncomprises: an electrode group in which a positive electrode and anegative electrode are spirally wound with a separator interposedtherebetween; and a non-aqueous electrolyte comprising a non-aqueoussolvent and a lithium salt dissolved in the non-aqueous solvent, thepositive electrode including a positive electrode material mixture layercomprising a nickel-containing lithium composite metal oxide, and ischaracterized in that a product of A and B equals 150 to 350, A equals15 to 20%, and B equals 10 to 25%, where A (%) represents a porosity ofthe positive electrode material mixture layer, and B (%) represents avolume percentage of ethylene carbonate in the non-aqueous solvent.

The nickel-containing lithium composite metal oxide used in thisinvention is preferably represented by a formula: LiNi_(x)M_(y)L_(z)O₂,where M is at least one selected from Co and Mn, L is at least oneselected from the group consisting of Al, Mg, Ca, Si and Ti, 0.3≦x≦0.9,0.1≦y≦0.5, and 0.005≦z≦0.1.

Electrolytes for use in non-aqueous electrolyte secondary batteries areusually mixtures of a cyclic carbonate such as ethylene carbonate and achain carbonate. Cyclic carbonates have a high dielectric constant andhigh viscosity whereas chain carbonates have a low dielectric constantand low viscosity. Accordingly, in order to yield an electrolyte havinghigh ion conductivity, they are usually mixed such that the volume ratioof cyclic carbonate and chain carbonate is 1:4 to 5:5 and that thevolume percentage of ethylene carbonate is 20 to 40%.

Investigations made by the present inventors have revealed that, in apositive electrode whose active material is a nickel-containing oxide,the correlation between the porosity of the material mixture layer andthe amount of ethylene carbonate has a significant effect on battery'ssafety. This can be explained as follows.

The positive electrode material mixture layer containing anickel-containing oxide has the disadvantage that it easily separatesfrom a current collector. This is presumably caused by a film derivedfrom ethylene carbonate which is formed on the surface of thenickel-containing oxide. More specifically, the film formed on thesurface of the active material impairs the adhesion between the activematerial and the current collector, resulting in easy separation of thematerial mixture layer. Further, the inventors found that thenickel-containing oxide readily forms a film derived from ethylenecarbonate upon contact with ethylene carbonate. This is presumablybecause a ring-opening and polymerization of ethylene carbonaterepresented by the following formula is repeated (see D. Aurbach et al.,J. Electrochem. Soc., 147(4), 1322-1331 (2000)):

LiNiO₂+(CH₂O)₂C═O→NiO₂—CH₂CH₂OCO₂Li.

This reaction occurs only in a nickel-containing oxide, and it does notoccur in other oxide such as a cobalt-containing oxide.

If a positive electrode material mixture layer separates from a positiveelectrode current collector when a nail is penetrated through a battery,the positive electrode current collector becomes bare around the areathrough which the nail is penetrated, allowing the current collector toeasily come into contact with a negative electrode active materialthrough the nail. Because the positive electrode current collector has alow resistance value, a large current flows intensely to the shortedarea, and the battery enters a heating mode, generating excessive heat.Accordingly, in a battery including a nickel-containing oxide, it isnecessary to prevent excessive heat generation by minimizing the amountof ethylene carbonate so as to prevent the formation of a film derivedfrom ethylene carbonate on the surface of the positive electrode activematerial layer, in other words, to prevent the separation of thematerial mixture layer.

It is also effective to form a structure whereby the positive electrodematerial mixture layer does not easily separate from the currentcollector. Specifically, when the positive electrode current collectorhaving a paste for forming positive electrode material mixture layerapplied thereto and dried is subjected to a rolling step, the degree ofrolling should be increased so as to reduce the porosity of the positiveelectrode material mixture layer. This embeds (or sinks) the activematerial in the current collector.

In view of the above, in the present invention, in a high capacitynon-aqueous electrolyte secondary battery whose positive electrodeactive material is a lithium composite metal oxide containing nickel asan essential element, both the porosity of the positive electrodematerial mixture layer and the amount of ethylene carbonate contained inthe electrolyte are adjusted to appropriate levels as described above,whereby the separation of the positive electrode material mixture layerfrom the positive electrode current collector is prevented. Accordingly,even if a short-circuit occurs as in nail penetration test, the contactbetween the positive electrode current collector and the negativeelectrode active material can be prevented, and thus excessive heatgeneration of the battery can be prevented.

The positive electrode active material preferably comprises secondaryparticles comprising agglomerated primary particles and havingprotrusions on the surface of the secondary particles, so as to allowthe positive electrode active material to easily embed itself in thepositive electrode current collector.

According to the present invention, in a high capacity non-aqueouselectrolyte secondary battery whose positive electrode active materialis a lithium composite metal oxide containing nickel as an essentialelement, the separation of the positive electrode material mixture layerfrom the current collector, as well as the contact between the positiveelectrode current collector and the negative electrode active material,can be prevented.

By decreasing the volume percentage of ethylene carbonate contained inthe non-aqueous solvent as the porosity of the positive electrodematerial mixture layer is increased, ignition and combustion can beprevented.

As a result, even if a nail is penetrated through the battery, excessiveheat generation can be prevented, thereby providing a high capacitynon-aqueous electrolyte secondary battery having improved quality.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

DETAILED DESCRIPTION OF THE INVENTION

A non-aqueous electrolyte secondary battery of the present inventioncomprises an electrode group in which a positive electrode, a negativeelectrode and a separator for separating the positive and negativeelectrodes from each other are spirally wound. The electrode group ishoused, along with a non-aqueous electrolyte, in a battery case having apredetermined shape. The shape of the battery case is not specificallylimited. Commonly used are cylindrical and prismatic battery cases. Whenthe battery case has a cylindrical shape, the electrode group should beformed into a column. When the battery case has a prismatic shape, theelectrode group should be formed to have a substantially oval crosssection.

The positive electrode comprises a positive electrode material mixturelayer and a positive electrode current collector carrying the materialmixture layer. The positive electrode current collector is preferably astrip-shaped metal foil (e.g., aluminum foil). The positive electrodematerial mixture layer is carried on each surface of the positiveelectrode current collector except one portion of the current collector.The portion of the positive electrode current collector serves as a leadconnecting portion or current collecting portion. Because the positiveelectrode material mixture layer is not formed on the lead connectingportion or current collecting portion, the positive electrode currentcollector is bare. In order to achieve a smaller and lighter weightbattery, the positive electrode current collector preferably has athickness of 10 to 25 μm. The positive electrode material mixture layercomprises a positive electrode active material and a binder. Other thanthe positive electrode active material and the binder, the positiveelectrode material mixture layer may further comprise, for example, aconductive material.

The positive electrode material mixture layer preferably has a porosityof 15 to 20%. If the positive electrode material mixture layer has aporosity exceeding 20%, the degree to which the positive electrodeactive material particles are embedded in the current collector becomessmall. As a result, the positive electrode material mixture layerseparates easily from the positive electrode current collector,increasing the possibility of the bare portion of the current collectorcoming into contact with the negative electrode active material. Inother words, the possibility of excessive heat generation increases. Ifthe positive electrode material mixture layer has a porosity of lessthan 15%, the charge/discharge performance becomes low, facilitating thedegradation of battery characteristics over charge/discharge cycles.

The higher the porosity, the more likely the positive electrode materialmixture layer will separate from the current collector, resulting inpoor safety. For this reason, it is necessary to ensure safety bypreventing the formation of a film derived from ethylene carbonate onthe surface of the positive electrode active material. To achieve thisend, it is preferred to reduce the volume percentage of ethylenecarbonate in the electrolyte. At the same time, because the batterysafety tends to increase as the porosity of the positive electrodematerial mixture layer is decreased, the volume percentage of ethylenecarbonate may be increased to ensure charge/discharge performance of thebattery as long as the volume percentage stays within an appropriaterange. As used herein, the “appropriate range” means 150≦A·B≦350, whereA represents the porosity (%) of the positive electrode material mixturelayer and B represents the volume percentage of ethylene carbonate inthe non-aqueous solvent.

A description is now given of a method for producing the non-aqueouselectrolyte secondary battery of the present invention.

(i) Preparation of Positive Electrode Material Mixture Paste

A positive electrode material mixture paste is first prepared containinga positive electrode active material, a binder, a conductive materialand a dispersing medium. The dispersing medium is preferablyN-methyl-pyrrolidone (hereinafter simply referred to as NMP).Alternatively, a ketone such as acetone may be used. When the ketone isused, it is preferably used together with NMP.

The amount of the conductive material contained in the positiveelectrode material mixture paste is preferably 1 to 3 parts by weightper 100 parts by weight of the positive electrode active material. Whenthe amount of the conductive material is not less than 1 part by weight,the decrease of electron conductivity of the positive electrode can beprevented, and thus the cycle life of the battery can be extended.Likewise, when the amount of the conductive material is not greater than3 parts by weight, the decrease of battery capacity can be prevented.

(ii) Production of Positive Electrode

The positive electrode material mixture paste is applied onto bothsurfaces of a current collector serving as the positive electrode coremember. The applied films are dried and rolled, whereby positiveelectrode material mixture layers are formed integrally with the currentcollector. The current collector having the positive electrode materialmixture layers formed thereon is cut into a predetermined size. Thereby,a positive electrode is produced. The total thickness of the currentcollector and the positive electrode material mixture layers formed onboth surfaces of the current collector is usually 80 to 200 μm.

(iii) Production of Negative Electrode

The method for producing the negative electrode is not specificallylimited, and it may be a conventional method. For example, a negativeelectrode material mixture paste is first prepared containing a carbonmaterial capable of absorbing and desorbing lithium ions and a binder.The negative electrode material mixture paste is then applied onto bothsurfaces of a current collector serving as the negative electrode coremember. The applied films are dried and rolled, whereby negativeelectrode material mixture layers are formed integrally with the currentcollector. The current collector having the negative electrode materialmixture layers formed thereon is cut into a predetermined size. Thereby,a negative electrode is produced. The total thickness of the currentcollector and the negative electrode material mixture layers formed onboth surfaces of the current collector is usually 80 to 200 μm.

The binder for use in the negative electrode material mixture can be astyrene-butadiene copolymer (SBR), core-shell rubber particles, or fineparticles of a polymer containing a polyacrylic acid unit. In order toimpart favorable viscosity to the negative electrode material mixturepaste, carboxymethyl cellulose or polyethylene oxide may be added. Theamount of the binder contained in the negative electrode materialmixture is preferably 1.5 to 4 parts by weight per 100 parts by weightof the carbon material. The negative electrode current collector can bea metal foil such as a copper foil. In order to achieve a smaller andlighter weight battery, the negative electrode current collectorpreferably has a thickness of 8 to 20 μm.

(iv) Assembly of Battery

A battery is then assembled using the positive electrode, the negativeelectrode and a non-aqueous electrolyte. First, the positive electrodeand the negative electrode are spirally wound with a separatorinterposed therebetween to form an electrode group. During this step, ifthe electrodes and the separator are spirally wound into a cylinder, anelectrode group for cylindrical battery is obtained. If the electrodesand the separator are spirally wound so as to have a substantially ovalcross section, an electrode group for prismatic battery is obtained. Theobtained electrode group is inserted into a battery case having apredetermined shape. A non-aqueous electrolyte is then injected into thebattery case accommodating the electrode group. The opening of thebattery case is then sealed. Thereby, a non-aqueous electrolytesecondary battery is produced.

The separator is preferably a microporous film made of polyolefin suchas polyethylene or polypropylene. The separator usually has a thicknessof 10 to 40 μm.

The non-aqueous electrolyte is not specifically limited, and anyconventional electrolyte employed for non-aqueous electrolyte secondarybatteries can be used. Normally and preferably used is an electrolyteprepared by dissolving a lithium salt in a non-aqueous solvent. Thelithium salt can be, for example, LiPF₆, LiBF₄ or the like. They may beused singly or in a combination of two or more.

The non-aqueous solvent can be ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or thelike.

The non-aqueous electrolyte may further contain an additive forenhancing resistance to overcharge. A preferred example of such additiveis a benzene derivative comprising a phenyl group and a hydrocarboncyclic compound group adjacent to the phenyl group. Examples of suchbenzene derivative include biphenyl, cyclohexylbenzene, diphenyl etherand phenyl lactone.

The present invention is described in further detail below withreference to examples, but it should be understood that the presentinvention is not limited thereto.

EXAMPLE 1

(i) Production of Positive Electrode

A lithium nickel composite oxide (composition formula:LiNi_(0.8)Co_(0.15)Al_(0.05)O₂) was used as the active material. Apositive electrode material mixture paste was prepared according to theprocedure described previously.

This positive electrode material mixture paste was applied onto bothsurfaces of a 15 μm thick current collector made of aluminum foil, whichwas then dried and rolled to form positive electrode material mixturelayers integrally with the current collector. The resultant currentcollector is then cut into a size of 56 mm in width and 610 mm inlength. Thereby, a positive electrode was produced. The total thicknessof the aluminum foil and the positive electrode material mixture layersformed on both surfaces of the aluminum foil was 159 μm. The positiveelectrode material mixture layers had a porosity of 20%.

(ii) Production of Negative Electrode

A negative electrode material mixture paste was prepared by mixing 100parts by weight of spherical natural graphite powder with 1 part byweight of BM-400B (trade name) (core-shell rubber particles containingan acrylonitrile group in the core and having a styrene groupincorporated in the shell) available from Zeon Corporation, Japan, 1part by weight of carboxymethyl cellulose and an appropriate amount ofwater.

This negative electrode material mixture paste was then applied ontoboth surfaces of a 10 μm thick current collector made of copper foil,which was then dried and rolled to form negative electrode materialmixture layers integrally with the current collector. The resultantcurrent collector is then cut into a size of 58 mm in width and 640 mmin length. Thereby, a negative electrode was produced.

(iii) Assembly of Battery

The positive electrode produced above was allowed to sit for one day.Thereafter, the positive electrode and the negative electrode werespirally wound with a 20 μm thick separator made of polypropyleneinterposed therebetween to form a columnar electrode group. Theelectrode group was then inserted into a cylindrical bottomed batterycase, after which a non-aqueous electrolyte was injected into thebattery case. The non-aqueous electrolyte used here was prepared bydissolving LiPF₆ in a non-aqueous solvent of ethylene carbonate (EC),dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) at a volumeratio of 10:20:70 at a LiPF₆ concentration of 1 mol/L. After theinjection, the opening of the battery case was sealed with a sealingplate and a gasket. Thereby, a cylindrical lithium ion secondary batteryhaving an outer diameter of 18 mm and a height of 65 mm was produced(nominal capacity: 2800 mAh).

EXAMPLE 2

A battery was produced in the same manner as in EXAMPLE 1 except thatthe proportions of DMC and EMC in the non-aqueous solvent were eachdecreased by 2.5%, and that the volume percentage of EC was changed to15%.

EXAMPLE 3

A battery was produced in the same manner as in EXAMPLE 1 except thatthe porosity of the positive electrode material mixture layer waschanged to 15%.

EXAMPLE 4

A battery was produced in the same manner as in EXAMPLE 1 except thatthe porosity of the positive electrode material mixture layer waschanged to 15%, that the proportions of DMC and EMC in the non-aqueoussolvent were each decreased by 5%, and that the volume percentage of ECwas changed to 20%.

EXAMPLES 5 AND 6

A battery of EXAMPLE 5 was produced in the same manner as in EXAMPLE 1except that LiCoO₂ was added to the positive electrode active materialin an amount of 10 wt %.

Likewise, a battery of EXAMPLE 6 was produced in the same manner as inEXAMPLE 2 except that LiCoO₂ was added to the positive electrode activematerial in an amount of 10 wt %.

COMPARATIVE EXAMPLE 1

A battery was produced in the same manner as in EXAMPLE 1 except thatthe proportions of DMC and EMC in the non-aqueous solvent were eachdecreased by 5%, and that the volume percentage of EC was changed to20%.

COMPARATIVE EXAMPLE 2

A battery was produced in the same manner as in EXAMPLE 1 except thatthe porosity of the positive electrode material mixture layer waschanged to 15%, that the proportions of DMC and EMC in the non-aqueoussolvent were each decreased by 7.5%, and that the volume percentage ofEC was changed to 25%.

COMPARATIVE EXAMPLE 3

A battery was produced in the same manner as in EXAMPLE 1 except thatthe porosity of the positive electrode material mixture layer waschanged to 10%, that the proportions of DMC and EMC in the non-aqueoussolvent were each decreased by 7.5%, and that the volume percentage ofEC was changed to 25%.

COMPARATIVE EXAMPLE 4

A battery was produced in the same manner as in EXAMPLE 1 except thatthe porosity of the positive electrode material mixture layer waschanged to 25%.

Evaluation

The batteries produced in EXAMPLEs 1 to 6 and COMPARATIVE EXAMPLEs 1 to4 were evaluated by the following tests.

(Nail Penetration Test)

Each battery was charged to 4.25 V, and then subjected to nailpenetration test. In the nail penetration test, the charged battery wasplaced horizontally. A nail made of stainless steel was penetratedthrough the center of the battery using a hydraulic press. The resultsare shown in Table 1.

(Cycle Test)

After assembled, each battery was subjected to cycle test in anenvironment of 25° C. in the following procedure. The results are shownin Table 1.

1) Constant current-constant voltage charge: each battery was charged ata constant current of 1960 mA until the battery voltage reached 4.2 V.Then, the battery was charged at a constant voltage of 4.2 V until thecurrent decreased to 140 mA.

2) Constant current discharge: the battery was discharged at a constantcurrent of 2800 mA until the battery voltage decreased to 2.5 V.

TABLE 1 Battery Active material Volume temperature (wt %) percentage innail Number Active Active Porosity of EC penetration of material 1material 2 A (%) B (%) A · B test cycles Ex. 1 100 0 20 10 200 83 510Ex. 2 100 0 20 15 300 113 620 Ex. 3 100 0 15 10 150 69 410 Ex. 4 100 015 20 300 115 590 Ex. 5 90 10 20 10 200 74 530 Ex. 6 90 10 20 15 300 82645 Comp. 100 0 20 20 400 140 690 Ex. 1 Comp. 100 0 15 25 375 132 650Ex. 2 Comp. 100 0 10 25 250 103 180 Ex. 3 Comp. 100 0 25 10 250 133 370Ex. 4 Note: Active material 1: LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ Activematerial 2: LiCoO₂

The results are discussed below.

When the positive electrode material mixture layer had a preferredporosity of 20%, favorable results were obtained. However, the batteryof COMPARATIVE EXAMPLE 1, in which the product of porosity A and volumepercentage B of EC in the non-aqueous solvent equaled 400, generatedexcessive heat in the nail penetration test. Although both the porosityand the volume percentage of EC were within the appropriate range,because the product thereof was large, the positive electrode materialmixture layer easily separated from the current collector. Accordingly,it is preferred to set the porosity A and the volume percentage B of ECsuch that the product A·B falls within a range from 150 to 350 as inEXAMPLEs 1 and 2.

When the porosity of the positive electrode material mixture layer wasreduced to 15%, similar to the above, the battery of COMPARATIVE EXAMPLE2, in which the volume percentage of EC was increased to 25%, generatedexcessive heat in the nail penetration test. In this comparativeexample, the porosity was reduced to prevent the positive electrodematerial mixture layer from separating from the current collector.However, because the volume percentage of EC was high, a thick film wasformed on the active material surface, so that the separation of thematerial mixture layer was not prevented. Accordingly, even when theporosity is reduced, it is preferred to set the volume percentage of ECsuch 150≦A·B≦350 is satisfied as in EXAMPLEs 3 and 4.

In COMPARATIVE EXAMPLE 3 in which the porosity of the positive electrodematerial mixture layer was reduced to 10%, excessive heat generation wasnot observed in the nail penetration test. This is presumably becausethe positive electrode active material was sufficiently embedded in thepositive electrode current collector, and the separation of the positiveelectrode material mixture layer was successfully prevented. As aresult, the positive electrode current collector and the negativeelectrode material mixture did not come into contact with each other inthe nail penetration test. However, the cycle characteristic ofCOMPARATIVE EXAMPLE 3 was extremely low. This is presumably because theporosity of the positive electrode material mixture layer was small, theactive material surface was not sufficiently wet with the electrolyte,resulting in a degradation in charge/discharge performance. From this,it can be seen that the porosity of the positive electrode materialmixture layer is preferably set to not less than 15%.

When the porosity was increased to 25% as in COMPARATIVE EXAMPLE 4, nomatter how much the volume percentage of EC was reduced, it did notprevent excessive heat generation in the nail penetration test. This ispresumably because the degree to which the active material was embeddedin the current collector was small, and therefore the positive electrodematerial mixture layer was in a condition easily separable from thecurrent collector. Accordingly, the porosity is preferably set to notgreater than 20%.

As is clear from the results of EXAMPLEs 5 and 6, the positive electrodeactive material is not necessarily a nickel-containing oxide alone, andmay further contain a small amount of LiCoO₂.

According to the present invention, it is possible to prevent excessiveheat generation of a non-aqueous electrolyte secondary battery resultingfrom the contact between the positive electrode current collector andthe negative electrode active material in the event of a short-circuit.The non-aqueous electrolyte secondary battery of the present inventionis useful as a power source for various electronics including cellphones.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A non-aqueous electrolyte secondary battery comprising: an electrodegroup in which a positive electrode and a negative electrode arespirally wound with a separator interposed therebetween; and anon-aqueous electrolyte comprising a non-aqueous solvent and a lithiumsalt dissolved in said non-aqueous solvent, said positive electrodecomprising a positive electrode material mixture layer comprising anickel-containing lithium composite metal oxide, wherein a product of Aand B equals 150 to 350, A equals 15 to 20%, and B equals 10 to 25%,where A (%) represents a porosity of said positive electrode materialmixture layer, and B (%) represents a volume percentage of ethylenecarbonate in said non-aqueous solvent.
 2. The non-aqueous electrolytesecondary battery in accordance with claim 1, wherein saidnickel-containing lithium composite metal oxide is represented by aformula: LiNi_(x)M_(y)L_(z)O₂, where M is at least one of Co and Mn, Lis at least one selected from the group consisting of Al, Mg, Ca, Si andTi, 0.3≦x≦0.9, 0.1≦y≦=0.5, and 0.005≦z≦0.1.